Sickle cell anemia is a recessive trait in humans.In a cross between a father who has sickle cell anemia and a mother who is heterozygous for the gene, what is the probability that their first three children will have the normal phenotype?
A)1/4
B)1/2
C)none
D)1/8
E)1/16 will be albino

Question

Quizplus · Accepted Answer

The father has two homozygous recessive alleles (ss) for sickle cell anemia and the mother has one dominant normal allele (S) and one recessive sickle cell allele (s). The Punnett square for their offspring is:

|  S  |  s  |
|:---:|:---:|
| ss  | Ss  |
| ss  | Ss  |

There is a 50% chance of each child inheriting the dominant normal allele from the mother (S), and a 50% chance of each child inheriting the recessive sickle cell allele from the father (s). Therefore, the probability of each child having the normal phenotype (SS or Ss) is:

P(S) = 0.5 + 0.5(0.5) = 0.75

The probability of having three children in a row with the normal phenotype is:

P(normal phenotype for 3 children) = (0.75)^3 = 0.42 or 1/8

Therefore, the answer is D.

Sickle Cell Anemia Is a Recessive Trait in Humans